This invention relates to a sensor and a method of sensing useful for determining the concentration of a component of interest. More particularly, the invention relates to such a sensor and method which employ certain grown metal-containing inorganic crystalline compositions and provide for determining the concentration of a gaseous component of interest.
In many situations it is highly desirable, even necessary, to know the concentration of one or more given components in a medium. A great many different sensors have been suggested. One class of sensors which have been found to be useful in a number of applications are the optical sensors. In general, an optical sensor involves a sensing element which is exposed to the medium or component to be analyzed, a source of light or other radiation to excite the sensing element, and a means to analyze the optical signal given off by the sensing element in response to the excitation energy. Optical fibers have been used as conduits for transporting the excitation energy to the sensing element and/or transporting the emitted signal from the sensing element for analysis. Certain sensors have the sensing element physically located directly adjacent the optical surface of the optical fiber.
To be effective, a sensor must provide accurate and reproducible concentration determinations. The term "concentration" refers to the amount per unit increment of measure, and/or partial pressure, and/or the presence or absence of a given component. The sensing element should be structurally and functionally stable in the medium being analyzed and at the conditions of use. In addition, since the sensor may be used repeatedly, or even continuously, over a long period of time, the sensor should have a response time which is compatible with the desired frequency of measurement. In other words, the sensor should quickly respond to changes in concentration of the component of interest so that such changes can be accurately reflected by the measurements given by the sensor. In summary, sensors, and in particular optical sensors, in which a sensing element is exposed to the medium to be analyzed, should have at least some and preferably all, of the following attributes: high sensitivity and selectivity, temperature insensitivity or compensation, good reversibility, fast response, highly stable response, infrequent need for calibration, good structural stability and should have the ability to tolerate corrosive and high temperature environments.
Zeolite molecular sieves are crystalline materials with a large and well defined interior volume. Access to this interior volume is controlled by openings, or pores, in the crystal. Molecules in the liquid or gas phase are adsorbed into the zeolite molecular sieves selectively on the basis of their size and polarity, among other things. Zeolite molecular sieves are aluminosilicates which contain charge balancing cations in the pore volume.
Arakawa, et al, in "Luminescence Properties of Eu Ion-Exchanged Y-type Zeolite", Journal of Luminescence, 20, (1979) 325-327, and in "Photoluminescence During the Catalysis of Water Decomposition on an Activated Europium (III) - Y Zeolite", J. C. S. Chem. Comm., 1979, 453-454, disclose excitation spectra of Eu.sup.3+ ion exchanged Y-zeolite in a vacuum. At higher temperatures under degassed conditions, Eu.sup.3+ is reduced to Eu.sup.2+, which has a characteristic spectrum Further, Arakawa, et al in "Physicochemical Studies of an Activated Europium Ion-exchanged Mordenite", Bull, Chem. Soc. Jpn., 57, 1290-1294 (1984); in "A Fluorescence Study of the Adsorption of Water, Methanol, and Acetic Acid on an Activated Europium Ion Exchanged Mordenite", Bull. Chem. Soc. Jpn., 57, 948-951 (1984); and in "Adsorption of Oxygen in an Activated Europium Ion-exchanged Mordenite", Mat. Res. Bull., Vol. 19, pp. 429-434, 1984, disclose studies of the fluorescence of Eu.sup.+2 ion exchanged onto mordenite in the presence of various materials, such as oxygen, water, methanol and acetic acid. In addition, Arakawa, et al, in "Luminescence Study of the Adsorption of Ammonia and Other Simple Molecules on an Activated Europium Ion Exchanged Mordenite", Inorg. Chem. 1985, 24, 3807-3810, disclose the measurement of the luminescence of the Eu.sup.2+ ion on a mordenite zeolite when exposed to ammonia and other simple molecules. Although the europium was ion exchanged into the molecular sieve in the 3+ oxidation state, Arakawa, et al converted the europium to the 2+ oxidation state before taking meaningful emissions data.
Suib, et al, in "The Coordination Environment of Eu (III) Ions in Hydrated A and Y Zeolites as Determined by Luminescence Lifetime and EXAFS Measurements", J. Chem Phys. 80(5)1984; and in "Framework Chemistry and Structure Involving Electron Transfer and Europium in Zeolites", Journal of Molecular Catalysis, 27, (1984) 71-80, disclose using measurements of luminescent lifetimes and EXAFS with Eu3+exchanged zeolites to determine the number of water molecules coordinated to the europium. Suib, et al do not measure emission intensity.
Strome and Klier, in "Effects of Carbon Monoxide Absorption of the Luminescence of Reduced Copper-Exchanged Y Zeolite", J. Phys. Chem. 1980, 84, 981-984; in "The Effects of Oxygen of Photo-luminescence and Resonance Energy Transfer in Copper (I) Y Zeolite", Synthetic Zeolites, Photoluminescence and Energy Transfer, 1980, American Chemical Society; and in "Photoluminescence and Resonance Energy Transfer in Zeolites", Adsorption and Catalysis on Oxide Surfaces, 1985, Elsevier Science Publishers B.V., Amsterdam, disclose luminescence emissions due to Cu.sup.1+ activator ions in Y-zeolites in the absence and in the presence of Ni.sup.2+, Co.sup.2+, or Mn.sup.2+ sensitizers. The effects of oxygen and carbon monoxide on the luminescence of copper exchanged Y-zeolite is also discussed. In effect, Strome and Klier were concerned only with emissions resulting from copper ions.
None of the known prior art suggests using sensing elements which include inorganic crystalline compositions grown on inorganic oxide compositions.
There continues to be a need for sensors and sensing methods useful to determine component, in particular gaseous component, concentrations.